This invention relates to an apparatus for generating ultrashort optical pulses by using the mode-locking technique of semiconductor lasers applied to optical computing, optical information processing, optical measurement, and optical communication.
The mode-locking by transverse magnetic (TM) mode oscillation of a semiconductor laser has been found more effective for ultrashort optical pulse generation than the mode-locking by transverse electric (TE) mode oscillation of a semiconductor laser widely employed hitherto.
To generate picosecond optical pulses using a semiconductor laser is an important technique for applications in optical computing, optical information processing, optical measurement and optical communication. As a known fact among-those skilled in the art, a mode-locking technique using a semiconductor laser with an external cavity configuration is known as one of the methods for generating ultrashort pulses. This technique may be roughly classified into active mode-locking techniques and passive mode-locking techniques. They are described in detail for example, by J. P. van der Ziel in "Mode Locking of Semiconductor Lasers," Semiconductor and Semimetals, vol. 22, Part B, Chapter 1, page 1, 1985. The active mode-locking technique modulates the injection current fed into the semiconductor laser at the round trip frequency of external absorber is contained inside the cavity, and the injection current is kept constant. In both cases, optical pulses are generated at the round trip frequency C/2L of the external cavity (where C is the velocity of the light, and L is the length of external cavity), that is, the time interval of 2L/C, and the pulse width of the optical pulse is from several picoseconds to several tens of picoseconds. Estimating from the envelope width of the oscillation spectrum, an optical pulse width which is nearly Fourier-transform-limited is obtained.
Besides, as mentioned above, for mode-locking using a semiconductor laser, the semiconductor laser has an external cavity configuration. Moreover, the external cavity side of the facet of the semiconductor laser is anti-reflection coated.
A basic composition of the prior art is shown in FIG. 1. An anti-reflection coating 18 is applied on a first end facet 14 of a semiconductor laser 16 composed of an active layer 10 responsible for stimulated emission under current injection, and first and second end facets 14 and 12 of cleaved facets of crystal. A laser light 20 emitted from this semiconductor laser 16 is reflected by an external reflector 22 disposed outside, and the reflected laser light 20 is fed back to the semiconductor laser 16. Since the anti-reflection coating 18 is applied on the first end facet 14, the cavity of this laser is composed of the second end facet 12 at the opposite side of the external cavity, and an external reflector 22. By setting the bias of the injection current into the semiconductor laser 16 below the threshold current, when current modulation is applied at the round trip frequency of the external cavity, the laser light 24 emitted from the second end facet 12 of the semiconductor laser 16 oscillates as an optical pulse train 26. This is a general active mode-locking arrangement.
In a conventional solitary semiconductor laser, the polarization of its output power oscillation in the TE mode in which the direction of electric field is parallel to the P-N junction of the active layer. By nature, the semiconductor laser can oscillate in the TM mode in which the direction of electric field of the output power is perpendicular to the P-N junction of the active layer, but always oscillates in the TE mode. This phenomenon is described in detail, for example, by T. Ikegami in "Reflectivity of Mode at Facet and Oscillation mode in Double-Heterostructure Injection Lasers," IEEE Journal of Quantum Electronics, vol-QE-8, No. 6, p. 470, 1982. Excerpts of the drawings in this publication are shown in FIG. 2. FIG. 2 shows that the two different orthogonally polarized light of the TE mode (solid line) and the TM mode (broken line) are completely different in the reflectivity at the semiconductor laser end facet; the abscisea axes denotes the thickness of the active layer of semiconductor, and the ordinate axis represents the intensity reflectivity of the laser end facet. The transverse mode is the 0th order fundamental mode oscillation, and the refractive index of the active layer is 3.6 and .DELTA.n shows the ratio of the refractive index of the cladding layer and active layer. As clear from FIG. 2, the reflectivity of the end facet is considerably higher in the TE mode as compared with the TM mode, although depending on the layer thickness and refractive index of the layer composing the laser. In other words, in the semiconductor laser in which the end facets are formed by cleavage, since the cavity reflectivity is greater in the TE mode than in the TM mode, only a small threshold gain is necessary for inducing stimulated emission, and the semiconductor laser oscillates in the TE mode. The same holds true not only in the solitary semiconductor laser but also in the mode-locked semiconductor laser, and the mode-locked semiconductor laser reported hitherto oscillates in the TE mode 28 as shown in FIG. 1.
On the other hand, controlling the polarization of the semiconductor laser emission light by inserting a polarization controller in the cavity of semiconductor laser has been recently studied. It is described in detail for example, by T. Fujita et al. in "Polarization switching in a single frequency external cavity semiconductor laser," Electronics Letters, vol. 23, p. 803, 1987, and by T. Fujita et al. in "Polarization bistability in external cavity semiconductor lasers," Applied Physics Letters, vol. 51, p. 392, 1987. In these publications, it has been shown that the semiconductor laser emission light oscillates in the TM mode. That is, as shown in FIG. 3, when, for example, a Glan-Thompson prism is inserted as a polarization controller 30 between the first end facet 14 and the external reflector 22, the laser light 32 inside the cavity oscillates in the TM mode 34. At this time, the polarization controller 30 is aligned to transmit only the TM mode of laser light. Therefore, the output of laser light 36 also oscillates in the TM mode.
In order to generate ultrashort optical pulses by mode-locking, it is necessary that the external cavity modes of the optical spectrum of the semiconductor laser be arranged at precisely equal frequency spacings, and that these many modes oscillate with the same phases. Accordingly, the reflectivity should be ideally zero at the emission end facet of the external cavity of the semiconductor laser composing the external cavity, that is, at the first end facet. In other words, as shown in the prior art in FIG. 1, an antireflection coating 18 is applied on the first end facet. Actually, however, the reflectivity at this end facet is not completely zero when the semiconductor laser oscillates in the TE mode, and it is reported that a certain residual reflectivity presents. If the residual reflectivity is present on the intermediate facet, the mode spacings of the external cavity modes of the optical spectrum of the oscillating laser light are not equal. The dependence of the external cavity mode spacings on the reflectivity of the intermediate facet has been closely discussed, for example, by H. Sato et al. in "Intensity fluctuation in semiconductor lasers coupled to external cavity," IEEE Journal of Quantum Electronics, vol. QE-21, p. 46, 1985. This phenomenon is described below while referring to FIGS. 4(a)-4(b). The optical spectrum of the conventional mode-locked semiconductor laser oscillating in the TE mode, as shown in FIG. 1, is shown in FIG. 4 (a), in which the relationship is .nu..sub.m+n+1 -.nu..sub.m+n .noteq..nu..sub.m+n -.nu..sub.m+n-1, where .nu..sub.m denotes the oscillation frequency of each external cavity mode, and m and n are integers. To realize an ultrashort pulse generation, the relationship .nu..sub.p+n+1 -.nu..sub.p+n .congruent..nu..sub.p+n -.nu..sub.p+n-1 as shown in FIG. 4 (b) is required. Only in this case, all the external cavity modes oscillate with the same phases and the laser generates the ultrashort optical pulses.
However, this ideal case has not been demonstrated yet. This is because, as mentioned earlier, the effect of the residual reflectivity at the intermediate facet of the external cavity semiconductor laser is great.
Therefore, as shown in FIG. 4 (b), when each external cavity mode of the mode-locked semiconductor laser oscillates at equal frequency spacings, it is possible for the first time to obtain the ultrashort optical pulses.
So far, in the short wavelength region, shorter than 0.6 .mu.m, as long as III-V compound semiconductor materials are used, laser oscillation is impossible due to the limit of the band-gap energy, and therefore there were no lasers oscillating with such a short wavelength, and large gas-lasers were used. Accordingly, the apparatus using the laser was very large in size, and the industrial field of application was limited. If a compact, short wavelength light source, for example, a light source in the blue wavelength region is realized, it will give an immensely large impact in the field of information processing such as optical disc and laser printer, and also in all optical measurements.
Recently studies have been hence concentrated on the conversion element for converting the semiconductor laser to a half wavelength by making use of the second harmonic wave generation, as reported, for example, by T. Taniuchi and K. Yamamoto in "Miniaturized light source of coherent blue radiation," Technical Digest of CLEO '87, WP6, 1987. FIG. 5 shows a configuration for converting the wavelength of the semiconductor laser light by an optical wavelength conversion element using a conventional LiNbO.sub.3 optical waveguide. A fundamental wave 50 oscillating in the TE mode emitted from a semiconductor laser 16 is collimated by a lens 52, and is converted into the TM mode by a half wave plate 54, focused by a lens 56, and enters an optical waveguide 60 fabricated on an LiNbO.sub.3 substrate 58. The fundamental wave propagating through the optical waveguide 60 is converted to the second harmonic wave 62 by Chelenkov radiation, when both phase velocities of the fundamental wave and the second harmonic wave are equal. At the present, a second harmonic wave of a wavelength of 0.42 .mu.m at about 1 mW is obtained at the optical output with 120 mW of semiconductor laser light input at a wavelength of 0.84 .mu.m.
In this prior art, the half wave plate 54 is used, which is used for converting the TE mode oscillation of the semiconductor laser emission light to the TM mode. The reason for converting to the TM mode is that only the TM mode of the semiconductor laser propagates effectively in the optical waveguide 60 fabricated on the LiNbO.sub.3.
It may lead to an idea of rotating the semiconductor laser by 90.degree. to regard the TE mode oscillation apparently as the TM mode oscillation, but such method is not recommended, which is explained in FIG. 6.
FIG. 6 shows the alignment of semiconductor laser and LiNbO.sub.3 optical waveguide, and the polarization direction of the electric field of laser light (shown as an arrow) and near field patterns (shown as ellipses) of the semiconductor laser at the laser facet, at the LiNbO.sub.3 facet when focused, and the near field pattern allowed to propagate in the LiNbO.sub.3 waveguide.
In line (a) of FIG. 6, the semiconductor laser and optical waveguide are simply aligned, so that the near field patterns are matched, but the semiconductor laser light does not propagate through the optical waveguide because the polarization direction is orthogonal and mismatched.
In line (b) of FIG. 6, by using a half wave plate as in the prior art in FIG. 5, the semiconductor laser light propagates through the optical waveguide, and an SHG light is obtained.
In line (c) of FIG. 6, the semiconductor laser is rotated by 90.degree.; at this time the polarization direction is matched, but the near field pattern is not matched, and favorable coupling is not obtained.
Therefore, as shown in line (d) of FIG. 6, by technically applying +.alpha. to the semiconductor laser, when the near field pattern and polarization direction are originally in the TM mode, the coupling efficiency increases, and the apparatus may be simplified.
In addition, the generation of ultrashort pulses has been reported by Taniuchi and Yamamoto in "Picosecond generation by blue laser light source using SHG," Collected Papers from the 49th Academic Seminar of Applied Physics Society 7a-ZD-8. In the configuration shown in FIG. 5, by the gain switching method of directly modulating the semiconductor laser by a comb signal generator, a second harmonic wave of an optical pulse width of about 10 psec is obtained.
The second harmonic wave subjected to Chelenkov radiation as mentioned in the prior art is collimated in the thickness direction of the waveguide, and is dispersed to a spreading angle of about 14.degree. in the transverse direction. Therefore, when a pin hole or the like is used in order to focus on a spot, most output is used in vain, and a higher output is needed for practical use. Therefore, what is important is to notably enhance the wavelength conversion efficiency of short pulse light and to further decrease the width of the short pulse light.